The present disclosure relates to a coordinate measuring device. One set of coordinate measurement devices belongs to a class of instruments that measure the three-dimensional (3D) coordinates of a point by sending a laser beam to the point. The laser beam may impinge directly on the point or on a retroreflector target in contact with the point. In either case, the instrument determines the coordinates of the point by measuring the distance and the two angles to the target. The distance is measured with a distance-measuring device such as an absolute distance meter or an interferometer. The angles are measured with an angle-measuring device such as an angular encoder. A gimbaled beam-steering mechanism within the instrument directs the laser beam to the point of interest.
The laser tracker is a particular type of coordinate-measuring device that tracks the retroreflector target with one or more laser beams it emits. Coordinate-measuring devices closely related to the laser tracker are the laser scanner and the total station. The laser scanner steps one or more laser beams to points on a surface. It picks up light scattered from the surface and from this light determines the distance and two angles to each point. The total station, which is most often used in surveying applications, may be used to measure the coordinates of diffusely scattering or retroreflective targets. Hereinafter, the term laser tracker is used in a broad sense to include also total stations.
Ordinarily the laser tracker sends a laser beam to a retroreflector target. A common type of retroreflector target is the spherically mounted retroreflector (SMR), which comprises a cube-corner retroreflector embedded within a metal sphere. The cube-corner retroreflector comprises three mutually perpendicular mirrors. The vertex, which is the common point of intersection of the three mirrors, is located at the center of the sphere. Because of this placement of the cube-corner within the sphere, the perpendicular distance from the vertex to any surface on which the SMR rests remains constant, even as the SMR is rotated. Consequently, the laser tracker can measure the 3D coordinates of a surface by following the position of an SMR as it is moved over the surface. Stating this another way, the laser tracker needs to measure only three degrees of freedom (one radial distance and two angles) to fully characterize the 3D coordinates of a surface.
One type of laser tracker contains only an interferometer (IFM) without an absolute distance meter (ADM). If an object blocks the path of the laser beam from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner. Some laser trackers contain only an ADM without an interferometer. U.S. Pat. No. 7,352,446 ('446) to Bridges et al., the contents of which are herein incorporated by reference in their entirety, describes a laser tracker having only an ADM (and no IFM) that is able to accurately scan a moving target. Prior to the '446 patent, absolute distance meters were too slow to accurately find the position of a moving target.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker can use the position of the light on the position detector to adjust the rotation angles of the mechanical axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) an SMR that is moved over the surface of an object of interest.
Angle measuring devices such as angular encoders are attached to the mechanical axes of the tracker. The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
Several enhanced laser tracker systems are available or have been proposed for measuring six, rather than the ordinary three, degrees of freedom. The six degrees of freedom include three position degrees of freedom and three orientation degrees of freedom. Herein, the term position will be used to describe the three position degrees of freedom of a 6DOF target, and the term orientation will be used to describe the three orientation degrees of freedom.
Exemplary six degree-of-freedom (6DOF) systems are described by U.S. Pat. No. 7,800,758 ('758) to Bridges et al., the contents of which are herein incorporated by reference in their entirety, and U.S. Published Patent Application No. 2010/0128259 to Bridges et al., the contents of which are herein incorporated by reference in their entirety. Devices of this type will be called 6DOF-capable laser trackers or 6DOF laser trackers for short. In contrast, laser trackers which do not have 6DOF measurement capability will be called simply laser trackers or trackers for short. Trackers can only measure the position of a retroreflector.
Other 6DOF laser tracker designs have been proposed or developed in the past. One such method described in U.S. Pat. No. 4,714,339 to Lau et al. involves mounting the retroreflector in a gimbal mechanism. An integrated position sensitive device (PSD) and servo mechanism enables the retroreflector to pivot automatically to track the incoming laser beam. The orientation is then measured by angular encoders and a tilt sensor, and the orientation data is transmitted from the target to the 6DOF laser tracker by radio.
One type of six-DOF target used in some prior art systems is an augmented target. An augmented target includes a retroreflector for position measurements, and a second independent system for orientation measurements. Examples of augmentation include light emitting diodes (LEDs), angular encoders, position sensitive devices (PSDs), motors, level sensors, and accelerometers. Augmented targets all involve at least two separate subsystems, one for position and at least one more for orientation. Augmented systems require compatible special purpose targets, and augmented targets from different manufacturers are not compatible with one another and therefore are not interchangeable. Such systems also cannot measure the orientation of a non-augmented target such as a retroreflector or a spherically mounted retroreflector.
A laser tracker system which incorporates 6DOF capability offers significant advantages to the user. For example, with a 6DOF laser tracker and a compatible 6DOF probe, surfaces which are not on a direct line of sight from the laser can be measured. This allows interior surfaces such as bore holes to be measured without the need to repeatedly reposition the laser tracker. Also, a 6DOF laser tracker can be used in tandem with a compatible laser line probe. This creates a measuring system which combines the long range of the laser tracker with the touchless measurement capability of the laser line probe. A 6DOF tracker can also be used to monitor the position and orientation of a robot arm, enabling the robot to perform at a higher level of accuracy than would otherwise be possible. Another possible advantage of a 6DOF laser tracker is the ability increase the accuracy of standard SMR position measurements by correcting for centering errors which occur when the vertex of the cube-corner retroreflector does not precisely coincide with the center of the metal sphere. Not all 6DOF designs provide this capability however.
Unfortunately, because of their reliance on augmented targets, existing 6DOF laser trackers require a target that is manufacturer-specific, bulky, heavy, complex, active and costly. If an application requires large numbers of targets, such systems are cost-prohibitive. If a target is lost or damaged, replacement is expensive. Because of the complexity of these targets, they are inherently less reliable than a simple cube-corner retroreflector. Training expense is increased due to the increased complexity. Because augmented targets from different manufacturers are incompatible and cannot be interchanged, users are often forced to purchase their 6DOF laser trackers from a single supplier, increasing their costs and limiting their options. Finally, augmentation methods inherently do not allow for the correction of centering errors in an SMR.
It is clear from the foregoing considerations that a practical alternative to augmented 6DOF targets would be highly desirable. One such alternative is based on imaging. The idea is to collect an image of the cube-corner retroreflector. In principle, the only feature in such an image is a set of three lines which are the projections of the three dihedral lines formed by the intersections of the cube-corner's three flat mirrors. Image analysis methods can be used to determine the plane angles of the three lines, and from these the orientation of the cube-corner can be determined.
An exemplary laser tracker system 5 illustrated in FIG. 1 includes a laser tracker 10, a retroreflector target 26, an optional auxiliary unit processor 50, and an optional auxiliary computer 60. An exemplary gimbaled beam-steering mechanism 12 of laser tracker 10 comprises a zenith carriage 14 mounted on an azimuth base 16 and rotated about an azimuth axis 20. A payload 15 is mounted on the zenith carriage 14 and rotated about a zenith axis 18. Zenith axis 18 and azimuth axis 20 intersect orthogonally, internally to tracker 10, at gimbal point 22, which is typically the origin for distance measurements. A beam of light 46 virtually passes through the gimbal point 22 and is pointed orthogonal to zenith axis 18. In other words, beam of light 46 lies in a plane approximately perpendicular to the zenith axis 18 and that passes through the azimuth axis 20. Outgoing beam of light 46 is pointed in the desired direction by rotation of payload 15 about zenith axis 18 and by rotation of zenith carriage 14 about azimuth axis 20. Motors steer the outgoing light beam by rotating tracker components about the azimuth and zenith axes, in a manner known in the art. A zenith angular encoder, internal to the tracker, is attached to a zenith mechanical axis aligned to the zenith axis 18. An azimuth angular encoder, internal to the tracker, is attached to an azimuth mechanical axis aligned to the azimuth axis 20. The zenith and azimuth angular encoders measure the zenith and azimuth angles of rotation to relatively high accuracy. Outgoing beam of light 46 travels to the retroreflector target 26, which might be, for example, an SMR as described above. By measuring the radial distance between gimbal point 22 and retroreflector 26, the rotation angle about the zenith axis 18, and the rotation angle about the azimuth axis 20, the position of retroreflector 26 is found within the spherical coordinate system of the tracker.
Outgoing beam of light 46 may include one or more wavelengths. For the sake of clarity and simplicity, a steering mechanism of the sort shown in FIG. 1 is assumed in the following discussion. However, other types of steering mechanisms are possible. For example, it is possible to reflect a laser beam off a mirror rotated about the azimuth and zenith axes. The techniques described herein are applicable, regardless of the type of steering mechanism.
Magnetic nests 17 may be included on the laser tracker for resetting the laser tracker to a “home” position for different sized SMRs—for example, 1.5, ⅞, and ½ inch SMRs. An on-tracker retroreflector 19 may be used to reset the tracker to a reference distance. In addition, an on-tracker mirror, not visible from the view of FIG. 1, may be used in combination with the on-tracker retroreflector to enable performance of a self-compensation.
The basic idea of the imaging approach is shown in FIG. 2, FIG. 3 and FIG. 4. In FIG. 2, cube-corner retroreflector 200 comprises three mutually perpendicular plane mirrors 231, 232 and 233 (also herein referred to as reflecting surfaces) which intersect along dihedral lines 211, 212 and 213 (also herein referred to as straight marks). The three dihedral lines intersect at vertex 221. In FIG. 3, the three dihedral lines 211, 212 and 213 of cube-corner 200 are projected onto sensor plane 301 (also herein referred to as a two-dimensional photosensitive array), resulting in three plane lines 201, 202 and 203. Referring finally to FIG. 4, the plane lines 201, 202 and 203 have plane angles 401, 402 and 403 respectively relative to X axis 405. These three angles change in a well-defined way as the orientation of the retroreflector changes. To put it differently, if the three plane angles are known, then the three orientational degrees of freedom of the retroreflector can be determined. (There is ambiguity in the roll angle, but this can be resolved in a straightforward way.) The specific case shown involves collimated illumination without optics. In some embodiments, a lens would be used to accomplish the projection.
Since the retroreflector is used to determine both position and orientation, the imaging method eliminates the need to augment the retroreflector with additional systems. With the imaging approach, a 6DOF target consists of nothing more than a cube-corner retroreflector. As a result, the 6DOF target has all the desired properties: it is small, light, simple, passive, interchangeable and inexpensive. It also makes it possible to correct for centering errors in an SMR. The challenge is to get the desired range, speed and accuracy from an imaging approach.
Existing well-known methods for analyzing images can in principle be used to identify lines and extract their angles. Such methods include the Hough transform, the closely related Radon transform, edge detection methods and convolution methods. Unfortunately, existing image analysis methods are not well suited to the very specific application of finding three angles of three lines in the image of a cube-corner retroreflector to high precision. For one thing, these are all general purpose algorithms which do not take advantage of the prior knowledge that there are always exactly three lines in the image. They also do not take advantage of the fact that a retroreflected image has a very useful property, namely reflection symmetry. In other words, for each point in the image, there is a unique point on the exact opposite side of the cube-corner vertex which is its symmetry partner. In the ideal case, these two points must have exactly the same brightness. Finally, they do more computational work than is needed for this specific application. They solve for both the angles and the positions of the three lines, when only the angles are required for determining the orientation. This means that they are solving for six unknowns rather than three. This makes the computational load much greater than necessary, both in terms of data storage requirements and in terms of the number of arithmetic operations required.
The application of interest here is high speed precision dimensional metrology. In order to accurately measure moving targets, a frame rate on the order of 100 frames per second or more is typically required. Such high data rates are only feasible if the probability of an outlier or an outright failure in any given frame is extremely low. Unfortunately, existing image analysis methods do not meet this requirement. For example, it is well known that the Hough and Radon methods are prone to large errors when the image is degraded by real world effects such dirt in the optics. Since laser trackers are often used in dirty manufacturing environments, this is a serious limitation. Another pathology in real world images is diffraction effects: the image of a cube-corner retroreflector under coherent illumination may include spurious lines which are caused by diffraction. These lines are parallel to the desired lines but displaced from them. Existing methods are not designed to address this pathology. Another limitation of existing methods is insufficient precision. This is true in particular of edge detection and convolution methods. Because of the excessive data storage requirements and excessive numbers of arithmetic operations, existing methods of image analysis are often too slow for the application. Furthermore, existing general purpose image analysis algorithms do not lend themselves to cost effective implementations in special purpose high speed processors such as field programmable gate arrays (FPGAs), application specific integrated circuits (ASICs), digital signal processors (DSPs) and so on, precisely because of their general nature. Finally, existing methods do not perform well with images that are unfocused or defocused.
In summary, what is needed is a 6DOF laser tracker design that uses imaging to gain the advantages of a non-augmented target but avoids the limitations of existing image analysis methods.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.